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Gentamicin resistance in gram-negative bacilli: occurrence of modifying enzymes and their influence on susceptibility
Abstract:
Among 272 gram-negative bacilli, regarded as strains with reduced susceptibility to gentamicin by the disc test, 156 strains were resistant (greater than or equal to f microgram/ml) by the agar dilution method. Altogether 76% of the 156 strains were cross-resistant to tobramycin, kanamycin, streptomycin and amikacin. Amikacin showed the highest antibacterial activity in vitro. 32 of the resistant strains produced acetylating or adenylylating enzymes, which caused resistance. The production of enzymes was not limited to any genera. Complete cross-resistance to aminoglycoside antibiotics was recorded in 45%, 98% and 22% respectively, of Pseudomonas aeruginosa, Ps. maltophilia and other gram-negative rods. Enzyme-producing strains were not cross-resistant to all aminoglycosides. Changes in MICs and inhibition zones with different inocula among enzyme-producing strains emphasize the importance of standardized inocula in susceptibility testing.
Insights
Many gram-negative bacilli exhibit resistance to gentamicin, with significant cross-resistance to other aminoglycoside antibiotics. Enzyme production is a key factor in this resistance, highlighting the need for standardized susceptibility testing.
Area of Science:
- Microbiology
- Antimicrobial Resistance
Background:
- Gentamicin resistance is a growing concern in gram-negative bacilli.
- Aminoglycoside antibiotics are crucial for treating infections caused by these bacteria.
Purpose of the Study:
- To investigate the prevalence and mechanisms of gentamicin resistance in gram-negative bacilli.
- To evaluate cross-resistance patterns among various aminoglycoside antibiotics.
- To assess the role of enzymatic modification in aminoglycoside resistance.
Main Methods:
- Disc diffusion and agar dilution methods were used to determine gentamicin susceptibility.
- Cross-resistance to tobramycin, kanamycin, streptomycin, and amikacin was assessed.
- Enzyme production (acetylating or adenylylating) was identified as a resistance mechanism.
Main Results:
- 156 out of 272 gram-negative bacilli strains showed resistance to gentamicin by agar dilution.
- 76% of resistant strains exhibited cross-resistance to other aminoglycosides.
- Amikacin demonstrated the highest in vitro activity.
- Enzyme production was observed in 32 resistant strains, not limited to specific genera.
- Pseudomonas maltophilia showed the highest rate (98%) of complete cross-resistance.
Conclusions:
- Enzyme-mediated resistance significantly contributes to aminoglycoside cross-resistance in gram-negative bacilli.
- Standardized inoculum standardization is crucial for accurate susceptibility testing due to variable MICs and inhibition zones.
- Amikacin may retain activity against some enzyme-producing strains, warranting further investigation.